The invention relates to a coupling membrane for transmitting a rotational moment between a drive shaft hub and a rotor blade holder, especially in a gimbal rotor of a helicopter. The rotor blade holder carries plural rotor blades extending respectively radially relative to the rotation axis so as to thereby form a rotor blade plane. Bearings connecting the rotor blade holder to the drive shaft allow the holder to tilt relative to the shaft, while the center lines of the bearing arrangement between the holder and the hub intersect each other on the axis of the drive shaft.
In general, various couplings are used for transmitting rotational moments to or from rotating components including drive shafts. The present application particularly relates to a so-called membrane coupling arranged between a drive shaft and plural rotor blades that are positioned radially about the drive shaft. The membrane coupling uses a membrane as the coupling element, i.e. a so-called coupling membrane. The rotor blades are rotatably arranged in a rotational plane, and the coupling membrane is arranged between the rotor hub of the rotor shaft and the holder structure that holds the rotor blades. Such a coupling membrane is especially applicable in technical structures in which the rotor blade plane must be tiltable relative to the rotational axis of the rotor shaft. Thus, the coupling membrane must be able to allow and follow this tilting motion of the rotor blade plane while still transmitting the rotational moment from the shaft to the rotor blades. A particular application in the field of helicopter technology is the generally known gimbal rotor also called a Cardan rotor.
Such a generally known gimbal rotor is used in a rotary wing aircraft, for example, and particularly in a helicopter. However, other applications for such a gimbal rotor arrangement include machine constructions in which a rotating shaft must be provided with an elastic compensating coupling that transmits a rotational moment while allowing axial relative motion. For example, any application in which a rotating drive shaft drives rotating rotor blades in which tilting of the blade plane is to be enabled can use such a gimbal rotor.
The development of the gimbal rotor was especially motivated and driven in the field of helicopter technology by the on-going need to reduce or avoid the vibrations resulting from various rotor blade movements. First of all, an up and down cyclical or oscillating movement of each rotor blade takes place due to the different relative wind velocities as well as the cyclical blade pitch adjustments as each blade travels around a full circle. Further caused by the change of the rotor radius, the coriolis forces arising in this context in turn lead to a lead-lag pivoting or oscillation of each blade. These horizontal lead-lag motions as well as the known vertical flapping motions of the blades, which are both generally categorized as bending motions, generate vibrations that are conducted through the rotor head into the drive train and the fuselage construction. As a result, these vibrations influence not only the rotor construction itself, but also the entire fuselage construction.
The gimbal rotor provides the advantage in comparison to other rotors, that it significantly reduces or prevents the vibrations that arise due to the flapping and lead-lag oscillating of the rotor blades. This can be achieved because the individual flapping of the rotor blades is prevented and thus the resulting lead-lag oscillation of each blade is reduced or minimized. The gimbal rotor is one step in the direction toward a rotor that enables a tilting motion of the entire rotor blade plane relative to the drive shaft. A component that is necessary for the functioning of the gimbal rotor is the coupling membrane, which is subjected to extreme dynamic loads, which result from the double function of the coupling membrane. On the one hand, the rotational drive moment must be transmitted from the rotor mast to the rotor blades, and this requires a torsionally stiff and torsionally strong and rigid behavior of the coupling membrane. On the other hand, the tilting movements of the rotor must be enabled and followed while still maintaining the power transmitting connection between the mast and the rotor blades. This requires a relatively soft stiffness or flexibility characteristic of the coupling membrane with respect to the tilting movements of the rotor.
The above mentioned dynamic loads arising from the disparate functions, in effect, require diametrically opposed construction requirements for the coupling membrane. The necessary bending softness with respect to tilting movements of the rotor requires the coupling membrane to have a relatively small cross-section with relatively little material, which would, however, negatively influence the torsional stiffness, rigidity and strength that is also required of the coupling membrane. The coupling membranes that are known in the prior art suffer very rapid wear and degradation due to the strong dynamic loads. Especially the loads resulting from the transmission of the rotational moment lead to a relatively rapid material fatigue in the known coupling membrane constructions. Through the use of fiber reinforced composite materials, the operating life of known coupling membranes could be increased in an economical manner, but there is still room for improvement, especially in relation to the structure and configuration of the coupling membrane rather than the material of the coupling membrane.
In known rotor constructions, coupling membranes may have a convex and/or concave configuration and especially a complex dual convex and concave curvature. A disadvantage of such a structural configuration is that the double curvature of the surface of the coupling membrane causes difficulties in the fabrication thereof, and places limits on the use of fiber reinforced composite materials for making such coupling membranes. Moreover, such known concave and/or convex coupling membranes are not constructed in a fault tolerant or damage tolerant manner, so that a fault or damage of the coupling membrane may lead to a complete failure of the coupling arrangement.
It is also known to provide a gimbal rotor using a concave coupling membrane together with elastomeric transverse force or shear force bearings for a four-bladed rotor. The known concave coupling membrane in this context has a form similar to a V-belt pulley with a deep V-groove. The coupling membrane has a massive solid construction in order to achieve a sufficient torsional stiffness, rigidity and strength for transmitting the drive moment. That, however, is not particularly advantageous for achieving a bending softness or flexibility for the purpose of enabling, receiving, and transmitting bending moments arising from the movements of the rotor blades. Moreover, this known coupling membrane is not constructed in a redundant fashion, and does not provide fault or damage tolerance, so that upon the failure of the coupling membrane, no further drive of the rotor is possible.
In view of the above, it is an object of the invention to improve a gimbal rotor arrangement for transmitting the rotational drive moment from the rotor shaft to the rotor blades while achieving an improved bending elasticity and also providing failure reliability or tolerance. It is a further particular object of the invention to provide a configuration and construction of a coupling membrane that achieves a high torsional stiffness, rigidity and strength, in combination with a high bending flexibility for tilting relative to the rotor axis, with a simple and economical structure and fabrication thereof. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present specification.
The above objects have been achieved in an arrangement for transmitting a rotational moment between a drive shaft and plural rotor blades, especially in a gimbal rotor arrangement of a helicopter. The principle component of the arrangement is a coupling membrane arranged between the hub of the drive shaft and the rotor blade holder that carries the rotor blades extending radially relative to the rotation axis of the drive shaft. A bearing arrangement secures the rotor blade holder to the drive shaft in such a manner to allow tilting of the holder relative to the shaft, whereby the center lines of the bearing arrangement intersect at an intersection point on the rotation axis of the drive shaft.
Especially according to the invention, the coupling membrane comprises at least one plate-shaped element comprising a plurality of rings that are respectively arranged concentrically about a center point of the plate-shaped element. Throughout this specification, the term xe2x80x9cplate-shaped elementxe2x80x9d refers to an element having length and width dimensions in a major plane of the element that are substantially greater than the thickness of the element in a direction perpendicular to the major plane. For example, the length and width or lateral dimensions are at least 20 times the thickness dimension, or even significantly greater. Respective annular gaps are provided between the successive adjacent concentric rings, and the respective adjacent or neighboring rings are connected to each other by plural connector webs spanning across the annular gaps. According to one particular embodiment of the invention, each connecting web of a respective annular gap is displaced or offset in a circumferential direction relative to the respective connecting webs of the adjacent annular gap.
The rings may advantageously be circular or circular-arc-shaped, or be polygon-shaped. The rings may advantageously be arranged on a single plane, but it is also possible to arrange the rings on respective different planes, i.e. offset from one another on several planes. The plate-shaped element has an essentially constant and uniform thickness for all of the rings, while the respective rings may have different widths, although advantageously all the rings can have the same width. Any known securing means such as bolts, screws, pins, and the like can be used to secure the plate-shaped element to the rotor hub and to the rotor blade holder.
An advantageous grid-like structure of the plate-shaped element is achieved by arranging all of the connecting webs of a given annular gap respectively in pairs in such a manner so that respectively two paired connecting webs of a given annular gap together form a tapered or wedge-shaped V-configuration of the webs enclosing a vertex angle therebetween. Furthermore, each V-shaped or tapered pair of connecting webs of a given annular gap is arranged aligned along a radial line with a V-shaped or tapered pair of connecting webs of another one of the annular gaps. This arrangement provides a symmetry of the grid-like structure of the plate-shaped element. This embodiment is especially advantageous because it optimally takes up and transmits the forces acting on the plate-shaped element.
In an embodiment diverging from the above described embodiment, the plate-shaped element comprises at least three plate-segments that are circumferentially spaced by 120xc2x0 from each other along radial directions. Within a respective one of the plate segments, respectively one pair of connecting webs in a given annular gap is arranged aligned in a radial row or along a radial line with a pair of connecting webs of a different annular gap.
According to a further feature of the invention, it is possible to arrange two plate-shaped elements coaxially spaced apart from one another, so that these plate-shaped elements form respective parallel arranged coupling membranes. The two plate-shaped elements, i.e. the two coupling membranes, are connected, for example, between the radially outermost rings and between the radially innermost rings of the coupling membranes. In this manner, a failure-tolerant or fail-safe, reliable construction of the coupling membrane is achieved. The respective plate-shaped elements arranged parallel to each other may respectively each be plate-shaped elements that are not isotropic or quasi-isotropic in their structure. Rather, it is simply important that the overall arrangement of the plural plate-shaped elements together with each other is carried out in such a manner that the sum or total effect of the several plate-shaped elements forms a quasi-isotropic plate stack, i.e. a coupling arrangement that provides uniform stiffness characteristics around its entire circumference when the stack of plates is considered as a whole.
With respect to the bending flexibility, the present inventive coupling membrane has achieved a surfacially extending, plate-shaped, structurally integratable elastic (fictitious) bending joint that provides the necessary bending flexibility. The invention is able to achieve and satisfy the respective opposing requirements of a soft bending flexibility combined with a torsional and transverse or shear stiffness in a one-piece or integral plate as a coupling membrane. Furthermore, the configuration of a torsionally stiff and strong, yet also flexibly elastic plate having an advantageous grid-like pattern, further provides the possibility of a fault tolerant and damage tolerant arrangement of the coupling membrane or plate-shaped element in the rotor arrangement. A further advantage is the simple fabrication of a simple planar plate-shaped element. Also, the plate-shaped element achieves an improved strength.
As described above, the coupling membrane is formed from at least one plate-shaped element, but may include a plurality of plate-shaped elements to achieve a redundancy and failure tolerance. For the sake of simplicity, the plate-shaped element will also simply be referred to as a plate herein, or generally also as a coupling membrane. That makes sense, because the plate-shaped element or the plate is used as the coupling element, i.e. the coupling membrane. Generally, the plate-shaped element represents a surfacially extending plate, with openings that penetrate through the thickness of the material of the plate between the upper surface and the lower surface thereof. Thus, the plate is given a grid-like structure. Alternatively, however, the plate could be embodied only to have individual plate segments formed therein.
The present inventive plate as a coupling membrane provides a significant structural improvement in comparison to the above described conventional concave membranes. The inventive plate is stiff in the major plane of the plate, yet flexibly bendable in directions out of that major plane. Thus, the plate represents a rotationally stiff compensating coupling. The present plate does not have a complex double curvature as do the known concave membranes, so that the present inventive plate may readily be fabricated in an economical manner using fiber reinforced composite materials built up by fiberglass layers or plies. The fiber reinforced composite material is especially embodied in a failure tolerant and reliable manner by using carbon fiber layers or plies in addition to glass fiber layers or plies. Alternatively, the present plate may be fabricated of metal, completely instead of fiber reinforced composite material, or in addition to one or more layers of fiber reinforced composite material, whereby the fabrication using metal is also simplified.